1. Field of the Invention
The present invention relates to aqueous paper coating compositions containing latex as the sole adhesive and methods of coating using such compositions, especially the cast coating of paper and paperboard.
2. Description of the Prior Art
Paper coating compositions comprise two essential elements: a pigment and an adhesive or binder which bonds the pigment to the paper base. The cast-coating method of coating paper comprises applying an aqueous composition to a paper web or substrate; pressing the coated surface of the paper against a polished, heated, finishing drum; and maintaining the coated surface of the paper web in intimate non-slipping contact with the drum by adhesion until the coating has dried, whereupon the paper is separated from the drum. This process produces the mirror image of the finishing drum surface on the coated surface of the paper. As a result, the coated paper thus produced has very high gloss and may actually exhibit increased bulk over the uncoated basestock which results from the introduction of moisture into the swellable, fibrous bodystock during the coating and finishing steps.
In the cast-coating method as practiced by the present inventor, the surface of the finishing drum is higher than 100.degree. C, e.g., from 120.degree. to 150.degree. C, and the pressure in the nip where the coated paper is pressed against the finishing drum is greater than the vapor pressure of water at said temperature, all as taught by Hart in U.S. Pat. No. 2,919,205, assigned to the assignee of the present invention. Additionally, in employing the coatings of the present invention, a pool of boiling water is maintained in the casting nip. Steam evolving from this pool precludes air from the nip and prevents the formation of air bubbles in the coating. Moisture provided by the steam and boiling water also serves to rewet the coating just before contact with the casting drum, thus promoting good bonding of the coating to the drum surface. See U.S. Pat. No. 2,950,989 to Freeman assigned to the assignee of the present invention.
As a result of the inventions just described, cast coating speeds far faster than those taught by the prior art up to that time were made possible. This speed increase was due to the greater drying capacity available when casting drum temperatures were elevated above 100.degree. C. Hart increased nip pressures above the vaporization point of water at a given temperature in order to avoid too-sudden vaporization of water in the coating and basestock at these elevated temperatures and resultant violent disruption of the coated paper web. Shortly afterward, Freeman found unexpectedly that steam and boiling water could be utilized in the casting nip without causing excessive wetting and subsequent disruption of the cast paper surface. Although these techniques of Hart and Freeman permitted far faster operation (250 feet per minute or more) than was thought possible in the prior art, such extreme conditions of heat and pressure in the presence of nip moisture placed great physical demands upon the coating employed.
In order to obtain maximum benefits from the Hart and Freeman teachings, coatings were developed which were gelled or coagulated before reaching the casting nip and thus had greater resistance to the disruptive nip forces. In addition, such coatings were water resistant, and not subject to wash-off when brought into contact with a pool of boiling water in the casting nip. These gelled coatings have been of two basic types: acid-gelled, such as are described in U.S. Pat. No. 3,078,181, and heat-gelled, of the type taught by U.S. Pat. No. 3,356,517, both assigned to the assignee of the present invention. Such coatings have produced excellent end products when employed in conjunction with the teachings of the Hart and Freeman patents previously cited, and most recent research in cast coating has been done with gelled coatings of one type or the other.
Such gelled coating systems are not without disadvantages however. As set forth in U.S. Pat. No. 3,356,517, Column 2, lines 20-41, the acid-gel systems suffer from sensitivity of pH control and from the corrosive effects of the acid employed. The heat-gel systems recommended by this same U.S. Pat. No. 3,356,517 and those of U.S. Pat. No. 3,377,192 are themselves subject to difficulties, although of a different sort. Extremely careful control must be maintained over coating composition quantities and method of addition of components in order to make possible the desired gelation. Temperature must be controlled within narrow limits, both during coating formulation and application. Such coatings are even sensitive to ambient conditions, especially during hot, summer weather. The complexity of the coatings and order of addition necessitates long make-up times, with nearly constant attention from an operator. Such a time factor adds undesirably to labor costs.
For all of the foregoing reasons it would be highly desirable to eliminate the requirement for gelled coatings in the cast coating art, yet retain the advantages of high operating speeds which such coatings have made possible when employed in conjunction with high temperature and pressure conditions. Until the present invention, non-gelled coatings have been failures under the temperature and pressure conditions of high-speed casting for the reasons previously cited -- insufficient cohesiveness to withstand the disruptive forces in the nip, and solubility in the nip liquid. The present invention overcomes these difficulties and provides a coating composition which may be cast at speeds of 300 feet per minute or more at high temperature and pressure conditions but without the need for gelling the coating.
A further problem of the cast-coating art, encompassing both the earlier ungelled systems and the more recent gelled ones, has been the dependence of the art upon casein for successful casting. While as disclosed in the aforementioned U.S. Pat. Nos. 3,356,517 and 3,377,192, it is possible for the binder in a gellable coating composition to consist solely of latices it is still commercially preferred to employ a minor proportion of casein in the adhesive. Casein aids good bonding of the coating surface of the casting drum and it gives up water readily when heated. These characteristics prevent the formation of pits on the cast surface as moisture transpires through the back of the web while it is in contact with the casting drum. The use of casein, however, is attended by a number of drawbacks. One is that in order to make up a casein-based composition, it is necessary first to dissolve the casein in water which is then heated gently to a temperature not to exceed 60.degree. C. Careful control of the heating is required in order not to degrade the casein during the heating step. An alkali is then added to the casein solution, and the resulting solution is then added to a dispersion of the pigment.
Another disadvantage of systems containing casein, affecting gelled and ungelled systems alike, is that the price and supply of casein are subject to wide fluctuations. At present, casein for paper coating purposes is becoming virtually unobtainable. It is obvious that for ease of process operation and economic reasons it would be a great improvement in the cast-coating art if casein could be totally eliminated in all types of casting systems.
Attempts to replace the casein with a synthetic resin latex or emulsion have not heretofore met with success in ungelled cast-coating compositions because such formulations are not sufficiently water-loving to cast well on the drum; that is, the coating in its wet condition will not properly adhere to the surface of the finishing drum. In the language of the art, it will not wet out. This adhesion problem is worsened as operating speeds are increased and additional heat is necessitated to dry the coating partially prior to contact with the casting drum. Most latices cross-link or coalesce under the effects of such heat and the coated paper surface is even less hydrophilic when it reaches the casting drum after heating than it is at ambient temperature.
One type of coating composition which has successfully eliminated the need for casein in the paper coating art apart from cast coating is that embodying the use of the so-called alkali-swellable or alkali-soluble latices. See U.S. Pat. No. 3,513,121 granted May 19, 1970. This patent discloses a composition for coating paper comprising an aqueous dispersion of pigment, an adhesive composition comprising a blend of copolymer latices one of which is capable of being rendered soluble by alkali and one of which is not capable of being rendered soluble by alkali, and sufficient alkali to dissolve the alkali soluble latex. This coating composition, however, has not shown itself to be useful in cast coating since the binder component becomes coalesced in the alkaline medium prior to reaching the casting drum.
An adhesive composition for use in coating paper comprising a blend of latices, one of which is capable of being swelled by alkali and the other of which is insoluble and not swellable to any substantial extent in aqueous alkali has heretofore been proposed. See Canadian Pat. No. 862,188. Another patent which discloses the use of an alkali-swellable latex as an adhesive for a paper coating composition is U.S. Pat. No. 3,409,569. These patents teach, however, that in order to obtain the necessary coating viscosity sufficient alkali must be added to the paper coating composition to swell the latex. When such a coating composition is employed in the cast coating method of the present invention, the binder of the coating composition has shown a tendency to absorb water, swell, and thus coalesce prior to the time the coated web comes into contact with the heated casting drum surface. The coating therefore fails to cast when brought into contact with the finishing surface of the casting drum.